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On 22/05/2020 16:43, Rich Freeman wrote: |
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> On Fri, May 22, 2020 at 11:32 AM Michael <confabulate@××××××××.com> wrote: |
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>> |
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>> An interesting article mentioning WD Red NAS drives which may actually be SMRs |
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>> and how latency increases when cached writes need to be transferred into SMR |
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>> blocks. |
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> |
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> Yeah, there is a lot of background on this stuff. |
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> |
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> You should view a drive-managed SMR drive as basically a journaled |
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> filesystem/database masquerading as a virtual drive. One where the |
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> keys/filenames are LBAs, and all the files are 512 bytes long. :) |
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> |
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> Really even most spinning drives are this way due to the 4k physical |
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> sectors, but this is something much easier to deal with and handled by |
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> the OS with aligned writes as much as possible. SSDs have similar |
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> issues but again the impact isn't nearly as bad and is more easily |
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> managed by the OS with TRIM/etc. |
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> |
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> A host-managed SMR drive operates much more like a physical drive, but |
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> in this case the OS/application needs to be SMR-aware for performance |
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> not to be absolutely terrible. |
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> |
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What puzzles me (or rather, it doesn't, it's just cost cutting), is why |
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you need a *dedicated* cache zone anyway. |
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|
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Stick a left-shift register between the LBA track and the hard drive, |
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and by switching this on you write to tracks 2,4,6,8,10... and it's a |
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CMR zone. Switch the register off and it's an SMR zone writing to all |
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tracks. |
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|
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The other thing is, why can't you just stream writes to a SMR zone, |
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especially if we try and localise writes so lets say all LBAs in Gig 1 |
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go to the same zone ... okay - if we run out of zones to re-shingle to, |
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then the drive is going to grind to a halt, but it will be much less |
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likely to crash into that barrier in the first place. |
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|
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Even better, if we have two independent heads, we could presumably |
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stream updates using one head, and re-shingle with the other. But that's |
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more cost ... |
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|
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Cheers, |
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Wol |